The present invention relates generally to proximity sensing devices, and more particularly to magnetic proximity sensors which utilize dc fields.
Proximity sensors are designed to indicate the spatial proximity of a ferrous object or "target". Such proximity sensors are utilized to indicate, for example, the mechanical status of movable parts in machinery, to give an indication of when an elevator is at a predetermined level, and to indicate when a hatch or a door is opened or closed.
There are a variety of proximity sensing devices currently utilized in the art. These devices include mechanical contact switches, magnetically activated mechanical switches, photoelectric detectors, and ac electromagnetic detectors. Each of these proximity sensing devices has certain disadvantages. For example, the mechanical contact switches are susceptible to mechanical damage, corrosion, and fouling by dirt and water. They are not generally favored when reliable, low maintenance, and long life are primary considerations. The magnetically activated mechanical switches (such as magnetic reed switches) eliminate some of the above disadvantages because they avoid direct contact with machinery parts and can be hermetically sealed. However, such switches are difficult to immunize against the effects of shock and vibration. Photoelectric detectors, although having a variety of operational advantages, are not suitable for use in environments where the optical window may easily become fouled by dirt and grease or painted by maintenance personnel. The optical sensor used in such photo-electric detectors may also be rendered inoperative by smoke. This is a serious disadvantage when the detector is being used aboard a naval vessel to sense such items as the status of an elevator or the status of a hatch.
The ac electromagnetic detectors make use of the electromagnetic interaction between alternating magnetic fields and objects of high conductivity, or high magnetic permeability. Included among these detectors are metal locators, eddy current sensors, and variable reluctance devices. Although the operation of these sensors is uninhibited by the presence of non-ferrous insulating materials between the sensor and the object to be detected, the ac field will not easily pass through conductors because of the electromagnetic interaction. Thus, since the electromagnetic fields must be allowed to penetrate the sensor for target detection, the sensor cannot be shielded against interference by spurious electromagnetic fields or radiation from such sources as radio transmitters, radar, power lines, etc.
The present design is directed to a magnetic proximity sensor which utilizes a dc magnetic field in its sensing operation. The use of such a dc magnetic field is attractive because the quasi-stationary magnetic field penetrates unimpaired through conductors and insulators alike, and any nonferrous material is transparent to the field. In contrast to devices using alternating fields which must be allowed to penetrate for detection, a dc magnetic detector can be made virtually impervious to all but very low-frequency electromagnetic radiation by using conductive shielding.
A further advantage of the use of this type of sensor is that it has no moving parts, and thus can be more easily immunized against the effects of shock and vibration than magnetically actuated mechanical switches. Moreover, along with other magnetic sensors but in contrast to devices using optical windows, the operation of a dc magnetic proximity sensor is unimpaired by dirt, paint or grease. Additionally, since the source of the dc field may be a permanent magnet connections and can be made very strong compared to ac electromagnetic sources. Finally, the dc magnetic field is a vector quantity whose direction and sense is determined by geometry rather then source intensity. Thus, it is possible to use the directional properties of the field to determine the geometrical relationships between a magnetic target and the detector independently of the sensitivity of the detector or the strength of the source.